1. Field of the Invention
The invention relates to systems for communication between surface and downhole equipment, and more particularly to systems and methods for reliably communicating data between surface and downhole equipment over a power cable, whether the cable is relatively short, long, or of intermediate length.
2. Related Art
Systems that provide communication between surface and downhole equipment (equipment positioned within a wellbore) over power cables are known. These systems may use various technologies, but typically, a transmitter coupled to the downhole equipment modulates a carrier signal with a data stream and impresses the modulated signal onto the power cable. A receiver coupled to the power cable at the surface then detects the modulated signal and demodulates the data stream from the signal.
The data stream, which may be acquired from sensors, produced by downhole equipment, or otherwise generated, may be encoded and modulated onto the carrier signal in a number of ways. The data may, for example, be frequency modulated, amplitude modulated, burst modulated, frequency shift key modulated, phase shift key modulated, etc. The modulated carrier signal may then be impressed onto the power cable, such as through a direct tap to the power cable, or through a reactive coupling. The receiver can be obtain the modulated carrier signal from the power cable using similar means, and can demodulate the data stream from the signal using complementary demodulation techniques. Communications can be unidirectional or bidirectional between downhole and surface equipment.
Signals that are carried over power cables are subject to interference and attenuation in the cable. Signals on the cable resonate at frequencies that are inversely proportional to the cable length (well depth) and attenuate in direct proportion to the cable length. Conventional systems use a single carrier signal that has a single frequency which is normally chosen so that it is high enough to avoid resonant interference in shallower wells, but low enough to minimize attenuation in deeper wells. While the carrier frequency may be optimized for a particular cable length, it cannot be optimized for multiple lengths; so the system may operate properly in some wells, but not in others.
It would therefore be desirable to provide systems and methods that enable reliable communications on the power cable between surface and downhole equipment, regardless of the length of the power cable.